This invention relates to electrical resistance heating elements, and more particularly, to modular heat exchangers including molded heating assemblies.
Electrical resistance heating elements are available in many forms. A typical construction includes a pair of terminal pins brazed to the ends of a Nixe2x80x94Cr coil, which is then axially disposed through a U-shaped tubular metal sheath. The resistance coil is insulated from the metal sheath by a powdered ceramic material, usually magnesium oxide. While such conventional heating elements have been the workhorse for the heating element industry for decades, there have been some widely-recognized deficiencies. For example, galvanic currents occurring between the metal sheath and any exposed metal surfaces of a hot water tank can create corrosion of the various anodic metal components of the system. The metal sheath of the heating element, which is typically copper or copper alloy, also attracts lime deposits from the water, which can lead to premature failure of the heating element. Additionally, the use of brass fittings and copper tubing has become increasingly more expensive as the price of copper has increased over the years. What""s more, metal tubular elements present limited design capabilities, since their shape can not be significantly altered without losing performance.
As an alternative to metal elements, polymeric heating elements have been designed, such as those disclosed in U.S. Pat. No. 5,586,214. The ""214 patent describes a process of making a polymeric heater in which an inner mold is used having a plurality of threaded grooves for receiving a resistance wire. The assembly is first wound with a wire and thereafter injection molded with an additional coating of thermoplastic material, containing a large amount of ceramic powder for improving the thermal conductivity of the coating.
It has been discovered that injection molding a layer of thermoplastic material loaded with large amounts of ceramic powder can be difficult. The viscous polymeric material often fails to fill the mold details and can leave portions of resistance wire coil exposed. Additionally, there can be insufficient wetting between the over molded thermoplastic coating and the resistance wire, with minimal thermoplastic bonding between the inner mold and the over molded thermoplastic coating. This has led to failure of such elements during thermal cycling, since entrapped air and insufficient bonding create crack initiation sites. Crack initiation sites lead to stress cracks that can lead to shorts in emersion applications. Cracks and entrapped air also limit the heating element""s ability to generate heat homogeneously, which tends to create hot and cold spots along the length of the element.
Efforts have been made to minimize hot and cold spots and insufficient bonding between layers of plastic materials having electrical resistance heaters disposed between their layers. In U.S. Pat. No. 5,389,184, for example, a pair of thermosetting composite structures are bonded together using a heating element containing a resistance heating material embedded within two layers of thermoplastic adhesive material. The two thermosetting components are permitted to cure, and then while applying pressure to the joint, electrical energy is passed through the heating element sufficient to heat the joint to above the melting temperature of the thermoplastic adhesive material. This heat fuses the layers of the thermoplastic adhesive to join the thermosetting materials together. The heating element remains within the joint after bonding and provides a mechanism to reheat the joint and reverse the bonding process in the field. While these procedures have met with some success, there remains a need for a less expensive, and more structurally sound, electrical resistance heating element.
The thermoplastic injection molding process has existed for several years. The plastic molding process has evolved to a point where the standard is high quality detailed, complex shapes, and smooth aesthetic surfaces. In addition, injection molding using plastic part tooling and molding equipment has evolved into a precise science capable of mass producing high quality plastic products.
Typical injection molding processes require that molten plastic be shot into a tool at an extreme high velocity. It is the interaction between the viscosity of the molten plastic, the molding pressure, and the tool geometry that creates a high quality, high detailed plastic part. Another common practice of injection molding incorporates the use of rigid inserts (i.e. insert molding) such as threaded bosses and ancillary mechanical parts. The required material fill velocity and mold pressure, however, are not conducive to accurate placement of element precursors within complex designs. The inability to overcome the adverse effects of mold flow on precursor element placement has limited molded heated part geometries to primarily flat shapes.
Therefore, along with the need for a less expensive, and more structurally sound electrical resistance heating elements, there remains a need to better implement element precursors within molded contoured shapes. There also remains a need for modular heat assemblies capable of accommodating layers of varying functionality and producing a range of heat outputs.
A modular heating component according to the present invention includes a first molded section defining a first opening therethrough and a second molded section defining a second opening therethrough and mated to the first molded section. The first and second molded sections are mated to define an enclosed area between the molded sections and the first and second openings are aligned to form a fluid-tight passage through the modular heating component. An electrical resistance heating material is secured between the first and second molded sections in the enclosed area. The electrical resistance heating material forms a predetermined circuit path having a pair of terminal end portions.
The modular heating component of the present invention provides several benefits through its design and configurability. The component may be stacked to form a fluid heater assembly. The number of components in the assembly may vary depending upon the preferred use of the assembly, as well as the configuration of the components. The components may have different resistance values whereby different wattage outputs occur when the components are coupled to a power source. Also, individual components may be coupled to different powers sources or control devices, such that different layers of the assembly produce different heat outputs and/or operate under different control parameters. The expandability of the stackable assembly, therefore, provides a designer with a wide range of wattages, voltages, and controlling and sensing options, and thereby provides great design latitude for usage in the medical, food, and other industries. The modular heating components may also be manufactured with diameters from a few inches to several feet, thereby allowing small and large scale applications. For example, larger modular heating components may be used to assemble a residential water heater.
The above and other features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention which is provided in connection with the accompanying drawings.